1. Field of the Invention
The present invention relates to a circumferential confronting type motor, in which an armature core and a drive magnet are positioned opposite each other in the radial direction. More specifically, it relates to a thrust bearing mechanism that relatively regulates through magnetic action the movement in the axial direction of a rotor assembly and a stator assembly of the circumferential confronting type motor.
2. Description of Related Art
A variety of motors have been proposed that employ a thrust magnetic bearing mechanism in order to stabilize support in the thrust direction of a rotor assembly in a circumferential confronting type motor. FIG. 12 is a cross-sectional view of a prior art that employs a thrust magnetic bearing mechanism in a circumferential confronting type pneumatic dynamic pressure bearing motor used in deflection scanning devices that use polygon mirrors.
The pneumatic dynamic pressure bearing motor shown in FIG. 12 comprises primarily of a rotor assembly 1 and a stator assembly 2. The rotor assembly 1 is equipped with a generally cylindrical-shaped rotor 10 with a shaft hole 13, and a drive magnet 11 mounted on an outer circumference side of the rotor 10 via a magnet yoke 12. In the meantime, the stator assembly 2 is equipped with a fixed shaft 20, whose one end section is fixed to a motor frame 21 and which is inserted in the shaft hole 13 of the rotor 10, and an armature core 22, which is indirectly supported by the motor frame 21 and has a drive coil 23 wound around it; where an outer circumference surface of the armature core 22 and an inner circumference surface of the drive magnet 11 oppose each other in the radial direction across a predetermined gap.
A dynamic pressure bearing mechanism 4 is formed between an outer circumference surface of the fixed shaft 20 and an inner circumference surface of the shaft hole 13 formed in the rotor 10, and two blocks of radial dynamic pressure bearing sections 41 and 42 that comprise the dynamic pressure bearing mechanism 4 are formed on at least one of the outer circumference surface of the fixed shaft 20 or the inner circumference surface of the shaft hole 13. When a predetermined amount of current is supplied to the drive coil 23, electromagnetic action between the armature core 22 and the drive magnet 11 causes an optical deflecting polygon mirror 6 mounted on the rotor 10 to rotate, and an incoming laser beam is reflected off of the polygon mirror 6 and is deflection-scanned in a predetermined direction.
In the pneumatic dynamic pressure bearing motor having such a structure, a concave section 24 is formed at the tip section of the fixed shaft 20, and a ring-shaped fixed-side thrust magnet 31 is provided in the concave section 24. A rotation-side thrust magnet 32 is provided more interior in the radial direction than the fixed-side thrust magnet 31 in a manner confronted with the fixed-side thrust magnet 31. The fixed-side thrust magnet 31 and the rotation-side thrust magnet 32 are positioned so that their respective poles are placed opposite to the opposing poles of the other thrust magnet, and together they make up a thrust magnetic bearing mechanism 3. The magnetic attractive force or the magnetic repulsive force that is generated between the two thrust magnets 31 and 32 restrains the oscillation of the rotor 10 in the thrust direction.
However, in such a pneumatic dynamic pressure bearing motor, dust such as magnetic particles and/or dust generated by abrasion is sometimes attached, although in extremely minuscule amounts, to the surfaces of the thrust magnets 31 and 32, due to the fact that the fixed-side thrust magnet 31 and the rotation-side thrust magnet 32 are formed by mixing and kneading magnetic particles and a binder and by using such methods as compress molding or sintering. The dust is then carried by the air current inside the motor and moves into the dynamic pressure bearing mechanism 4, which is formed between the outer circumference surface of the fixed shaft 20 and the inner circumference surface of the shaft hole 13. Since the dynamic pressure bearing mechanism 4 normally has a bearing gap of several micrometers, once dust enters the bearing gap, so-called bums occur in the dynamic pressure bearing mechanism 4, which can lead to problems in the bearing life and cause major problems such as the motor failing to rotate.
This problem is not limited to motors that use pneumatic dynamic pressure bearings, and can equally occur in motors that use bearings in which the bearing and the shaft are supported in a relatively rotatable manner across a minuscule gap, such as oil dynamic pressure bearings and oil-impregnated sintered bearings.
In this type of motor, the thrust bearing mechanism 3 is provided to restrain the oscillation of the rotor 10 in the thrust direction, and rare earth magnets are normally used as the thrust magnets 31 and 32, since they require large magnetic attractive force in spite of their relatively small volumes. This can consequently lead to escalating parts cost, which then makes the entire motor expensive.
In view of the problems described above, the present invention provides an inexpensive circumferential confronting type motor that can reduce both the parts cost and the motor price by improving the structure of the thrust bearing mechanism such that an independent thrust bearing mechanism that supports the rotor can be eliminated. The present invention also provides a circumferential confronting type motor that can reduce the occurrence of bearing failures by preventing the dust that is generated in the thrust bearing mechanism from entering the bearing section.
In order to solve the above problems, a circumferential confronting type motor in accordance with an embodiment of the present invention comprises an armature core that has drive coils wound around its plurality of poles, and a drive magnet positioned opposite to the armature core in the radial direction, wherein the drive magnet has a plurality of divided magnetized sections, each with a magnetic center and formed separated from each other in the axial direction by a non-magnetized section. In one aspect, the magnetic centers of the respective plurality of divided magnetized sections are provided in symmetrical positions to a magnetic center in the axial direction of the armature core. Further, electromagnetic action between the drive magnet and the armature core causes the two to rotate relatively, while magnetic action between the plurality of divided magnetized sections and the armature core regulates their relative movements in the axial direction.
According to the present invention, due to the fact that the electromagnetic action to rotatively drive the rotor is generated by having the drive magnet and the armature core positioned opposite to each other, and to the fact that the oscillation of the rotor in the thrust direction is restrained by the magnetic action between the plurality of divided magnetized sections, which are formed separated in the axial direction, and the armature core, it is possible to eliminate an independent thrust bearing mechanism that independently supports the rotor in the thrust direction, and thereby reduce the parts cost and the motor price.
In the circumferential confronting type motor described above, it is preferable for the plurality of divided magnetized sections to be formed symmetrically to the magnetic center in the axial direction of the armature core.
With such a structure, since the divided magnetized sections are symmetrically shaped there is no change in the magnetic actions and effects, even when the posture of the drive magnet is inverted vertically. As a result, motor components can be shared, which can reduce the motor price even further.
Additionally, the drive magnet may comprise a first divided magnetized section and a second divided magnetized section that are provided separated in the axial direction by a non-magnetized section, and it is desirable for the direction of the attraction of the first divided magnetized section to the armature core and the direction of the attraction of the second divided magnetized section to the armature core to be in opposite directions.
According to such a structure, the drive magnet comprises two pieces, which is the minimum quantity required to achieve the purpose of the present invention, which are the first divided magnetized section and the second divided magnetizing section. Due to the fact that the attraction directions to the armature core are opposite for the two divided magnetized sections, the oscillation in the axial direction of the drive magnet and the armature core can be regulated relatively with a simple structure.
In addition, by forming the non-magnetized section with a non-magnetic material, i.e., non-magnetic materials such as resin, ceramic, non-magnetic metals, the interval between the divided magnetized sections can be maintained at a constant interval; and by forming the non-magnetized section with void space, the interval between the divided magnetized sections can be adjusted, so that optimum relative positions of the divided magnetized sections to restrain the oscillation of the rotor in the thrust direction can be obtained through such an adjustment.
Furthermore, by mounting the drive magnet on a circumference surface of a magnet yoke and by providing in the magnet yoke and abutting the end surface of the drive magnet a positioning section to position the drive magnet in the axial direction, the drive magnet can be positioned in the axial direction surely and accurately.
When the drive magnet and the armature core are arranged such that one is provided in a rotor assembly while the other is provided in a stator assembly, where the rotor assembly and the stator assembly are supported in a relatively rotatable manner by a radial bearing mechanism comprising a fluid dynamic pressure bearing or an oil-impregnated sintered bearing supported across a predetermined gap, the bearing can wear out or the rotor assembly can stop rotating suddenly when dust enters the gap in structures in which the stator assembly supports the rotor assembly across the predetermined gap, as in fluid dynamic bearings or oil-impregnated sintered bearings. However, according to the present invention, due to the fact that the radial bearing mechanism is provided more interior in the radial direction than the drive magnet, there is an extremely low risk of dust entering the gap, which can lengthen the life of the motor.
A circumferential confronting type motor according to the present invention comprises a rotor assembly with a ring-shaped drive magnet and a cylinder section in which a shaft hole is formed; a stator assembly with a fixed shaft that is inserted in the shaft hole and that supports the rotor assembly in a rotatable manner, and an armature core positioned opposite to the drive magnet in the radial direction; and a dynamic pressure bearing mechanism formed between an outer circumference surface of the fixed shaft and an inner circumference surface of the shaft hole, wherein the drive magnet has a plurality of divided magnetized sections, each with a magnetic center and formed separated from each other in the axial direction by a non-magnetized section. In one aspect, the magnetic centers of the respective plurality of divided magnetized sections are provided in symmetrical positions to a magnetic center in the axial direction of the armature core, and electromagnetic action between the drive magnet and the armature core causes the two to rotate relatively, while magnetic action between the plurality of divided magnetized sections and the armature core regulates the movement relatively in the axial direction of the rotor assembly.
According to the present invention, due to the fact that the electromagnetic action to rotatively drive is generated by having the drive magnet and the armature core positioned opposite to each other, and to the fact that the oscillation of the rotor in the thrust direction is restrained by the magnetic action between the plurality of divided magnetized sections and the armature core, it is possible to eliminate an independent thrust bearing mechanism that independently supports the rotor in the thrust direction, and thereby reduce parts cost and the motor price. In addition, dust attached to the drive magnet can be prevented from entering the dynamic pressure bearing mechanism, which makes it possible to sustain the bearing performance for a long time.
Other objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.